The London based group Reprieve claimed that more than 700 prisoners passed through Portuguese airspace en route to the U.S. military prison at Guantánamo Bay, Cuba, between 2001 and 2006. Aircraft ''associated with the CIA'' made at least 115 stopovers at Portuguese airports, it claimed.
Clive Stafford Smith, the group's legal director, said he had ''zero doubts'' that Portuguese authorities were complicit in the CIA operations.
Reprieve attorneys campaign against the death penalty and represent more than 30 detainees held at Guantánamo Bay.
Portugal's attorney general last year opened a criminal investigation into claims that CIA flights, some of them allegedly carrying terror suspects, made stopovers in the country. The investigation came after a Portuguese member of the European Parliament presented a dossier of allegations.
The government said at the time that authorities had not unearthed any evidence of CIA flights and would not investigate the matter further.
Portugal's previous conservative government was a vocal supporter of the U.S. policy in Iraq. It hosted a prewar meeting in Portugal's mid-Atlantic Azores Islands that brought together President Bush and the British and Spanish prime ministers at the time, Tony Blair and Jose Maria Aznar.
The conservative government lost a 2005 general election to the Socialist Party, which is still in power.
A European Parliament report last year named Portugal among 14 EU countries believed to have colluded with the CIA extraordinary renditions program.
